Recovery of NGL from various feed gases has become more and more economically attractive, and there are numerous process configurations and methods known in the art to increase NGL recovery from a feed gas. Typical examples include cryogenic expansion configurations and processes described in U.S. Pat. No. 4,157,904 to Campbell et al., U.S. Pat. No. 4,251,249 to Gulsby, U.S. Pat. No. 4,617,039 to Buck, U.S. Pat. No. 4,690,702 to Paradowski et al., U.S. Pat. N Campbell et al., U.S. Pat. No. 5,799,507 to Wilkinson et al., and U.S. Pat. No. 5,890,378 to Rambo et al.
However, while all of these processes exhibit relatively high NGL recovery, several difficulties still remain. Among other things, NGL recovery processes employing cryogenic expansion typically require the use of a turboexpander to provide the cooling of the feed gas for high propane or ethane recovery. Moreover, many known NGL recovery processes are designed to process a specific gas composition at specific inlet conditions. Consequently, when the feed gas composition changes, NGL recovery will typically be reduced and potential product revenue lost. In order to maintain a high NGL recovery, costly revamp of equipment of the existing unit is often required. In addition, dehydration costs are often relatively high in such configurations as the entire feed gas needs to be dried (e.g., with the use of molecular sieves) to avoid freezing out of water in the cryogenic section. Consequently, various optimizations have been developed. For example, Campbell et al. describe in U.S. Pat. No. 6,182,469 that dried feed gas is cooled in a heat exchanger using cold residue gas and side reboilers as depicted in Prior Art FIG. 1. The condensed liquids are then separated in a separator and fed to the demethanizer. Alternatively, as described by Sorensen in U.S. Pat. No. 5,953,935, an absorber may be added upstream of a demethanizer as depicted in Prior Art FIG. 2. In such configurations, the liquids from the feed separator and the absorber bottoms are fed to the demethanizer. To enhance NGL recovery, the absorber overhead is cooled and refluxed by chilling with the demethanizer overhead vapor.
In still further known configurations, as described in U.S. Pat. No. 6,244,070 to Lee et al. and U.S. Pat. No. 5,890,377 to Foglietta, the reboiler duties are integrated in feed chilling, and in these configurations, liquids from the intermediate separators are fed to various positions in the downstream demethanizer for NGL recovery. These processes also include various means of providing cooling to the NGL processes. Exemplary configurations according to Elliott and Foglietta are depicted in Prior Art FIGS. 3 and 4, respectively.
Such optimized configurations typically increase the NGL recovery to at least extent. However, significant process limitations nevertheless remain. Most significantly, as the liquids separated from the intermediate cooling steps are fed to the demethanizer, such configurations generally operate at best efficiency for a relatively specific and narrow range of feed gas compositions. Consequently, when the feed gas composition varies, in particular, when the feed gas contains more C5(+) components, NGL recovery will be significantly reduced and energy consumption will be increased (Typically due to the additional C5(+) component increasing the operating temperature of the NGL recovery unit, thereby resulting in a less efficient operation of the turboexpander and the demethanizer).
Therefore, although various configurations and methods are known to recover NGL from a feed gas, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide methods and configurations for improved NGL recovery.